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In light microscopy, what is the "real image"?

The first lens in this system(the one closest to the specimen), is the objective lens, and the second9closest to the eye) is the ocular lens, or eyepiece. The objective forms the initial image of the specimen called the real image. When this image is projected up through the microscope body to the plane of the eyepiece, the ocular lens forms a 2nd image, the "virtual image". This image will be converted to aretinal and visual image.

What is the range of magnifying power?

The magnifying power of the objective alone usually ranges from 4X to 100X, and the power of the ocular alone ranges from 10X to 20X. The total power of magnification of the final image formed by the combined lenses is a product of the separate powers of the 2 lenses.

What is the difference between the scanning and oil immersion objectives?

The total magnification of standard light microscopes can vary from 40X with the lowest power objective(scanning),to 2000X with the highest power objective(oil immersion).

What is resolving power?

It is the ability to distinguish magnified objects clearly. A property of resolution is resolving power. Resolution is the capacity of an optical system to distinguish or separate two adjacent objuects or points from one another. The human eye can resolve 2 small objects as separate points just as long as the two objects are no closer than 0.2mm apart.

What is the relationship between wavelength and resolution?

The shortest visible wavelengths are in the violet blue portion of the spectrum(400nm), and the longest are in the red portion(750nm). because the wavelength must pass between the objects that are being resolved, shorter wavelengths(in the 400-500nm range)will provide better resolution.

What is numerical aperture?

It is a mathamatical constant that describes the relative efficiency of a lens in bending light rays. the higher the numerical aperture, the better the resolution.

Fluorescence microscopy has its most useful applications in diagnosing infections caused by specific bacteria, protozoans and viruses. It is commonly used for Mycobacterium tuberculosis. TRUE/FALSE

TRUE.

How are light microscopes different from electron microscopes?

The EM forms an image with a beam of electrons that can be made to travel in wavelike patterns when accelerated to high speeds. These waves are 100,000 times shorter than the waves of visible light. Resolving power is a function of wavelength.

An electron gun aims its beam through a vacuum to ring-shaped electromagnets that focus this beam on the specimen. Specimens must be pretreated with chemicals and dyes, and cannot be viewed in a live state. TRUE/FALSE

TRUE.

What is a wet mount?

It consists of a drop or two of the culture placed on a side, and overlaid with a cover glass. It is quick and easy, but has disadvantages.

Fixation of some fixed cells(microbial) is performed with chemicals such as alcohol and formalin. TRUE/FALSE

TRUE.

How do dyes impart color to cells?

They do this by becoming affixed to them through a chemical reaction. They are classified as "basic dyes"(cationic), which have a positive charge, or "acidic dyes"(anionic) which have a negative charge.

Many cells, especially those of bacteria, have numerous negatively charged acidic substances and thus stain more readily with basic dyes. Acidic dyes tend to be repelled by cells, so they are good for negative staining. TRUE/FALSE

TRUE.

What is the difference between a positive and negative stain?

Most procedures involve a positive stain, in which the dye actually sticks to the specimen and give it color. A negative stain is the reverse. The dye does not stick to the specimen, but settles around its outer boundary, forming a silhouette.